The United States currently imports approximately two-thirds of its petroleum, 60% of which is used for producing transportation fuels. As global petroleum supplies diminish and in an effort to reduce the combustion of petroleum-based fuels believed by many to participate in climate change from greenhouse gas emissions, it has become increasingly important to find alternative, and preferably renewable, energy sources. The alternative energy sources explored to date include hydropower, solar power, wind power, nuclear power and bioenergy, among others.
Furthermore, in 2007, the Energy Independence and Security Act (EISA) was enacted, setting standards for vehicle fuel economy and including provisions promoting the use of renewable fuels. EISA establishes production requirements for domestic alternative fuels under the Renewable Fuel Standard that requires transportation fuel in the U.S. to contain a minimum of 36 billion gallons of renewable fuels, including advanced and cellulosic biofuels and biomass-based diesel, by 2022. While cellulosic ethanol is expected to play a major role in meeting these requirements, there are a number of next generation biofuels that can help reach that goal that are more sustainable than cellulosic ethanol.
One of the next generation biofuels with the potential to help the U.S. meet the RFS goals is biofuels derived from algae. As recognized in the Department of Energy's National Algal Biofuels Technology Roadmap (2010), which is incorporated by reference in its entirety herein, algal biofuels will play an important role in meeting these production requirements.
Algae are organisms that typically grow in aquatic environments using light and carbon dioxide (CO2) to produce biomass, which can be used as potential biofuels, foods, feeds, high-value bioactives and can be used in bioremediation or as nitrogen fixing biofertilizers. Algae are classified as either macroalgae or microalgae, the latter being recognized as potentially good sources for biofuel production because of their relatively high oil content and their rapid biomass production. For example, potential oil yields from certain algae are projected to be 60 times higher than from soybeans and about 5 times more than oil palm per acre of land on an annual basis. In addition, microalgae grow very quickly (biomass can double as quickly as every 3.5 hours) compared to terrestrial crops and can be produced on non-arable land and using non-potable saline or waste water. Moreover, oil content in microalgae can exceed 80% by weight of dry mass, although levels of 20-50% are more common. Consequently, microalgae have recently garnered much attention from researchers, industry and the public as a source of biodiesel. However, microalgae can also be used to produce several other types of renewable biofuels, such as methane produced by anaerobic digestions of algal biomass and photobiologically produced biohydrogen.
Algal biomass consists of carbohydrate, proteins, and lipids or natural oils. Most of the natural oil produced by microalgae is in the form of triglycerol. The production of biodiesel from microalgae generally follows the production of biodiesel from plant and animal oils in that triglycerides are reacted with methanol, i.e., transesterification, to produce methyl esters of fatty acids (biodiesel) and glycerol. The transesterification is catalyzed by acids, alkalides and lipase enzymes.
Current algae cultivation methods include photobioreactors, raceways and open ponds. A raceway, or open pond, is a closed system about one foot deep in which algae are cultivated in conditions nearly identical to their natural environment. The pond is designed in a raceway configuration in which a paddlewheel provides circulation and mixing of algal cells and nutrients. The algal culture is fed in front of the paddlewheel during daylight while broth is harvest behind the wheel upon completion of a circulation loop. Because open ponds/raceways are open-air systems, they lose a substantial amount of water to evaporation and do not efficiently use CO2. In addition, raceways often suffer contamination from undesirable algae and microorganisms and can suffer low biomass concentration due to dark zones below the surface and initial algae layer of the pond. Finally, optimal culture conditions can be difficult to maintain and recovery of the algal biomass is expensive.
Another current method of culturing algae is through the use of photobioreactors, which were pursued to overcome the contamination and evaporation problems associated with open ponds. A typical, tubular photobioreactor has a number of transparent tubes usually less than 10 cm oriented to maximize sunlight capture. Microalgal broth is circulated from a reservoir to the solar collectors (tubes) and back to the reservoir, with a portion of the algal being harvested after the solar collection tubes. Because a photobioreactor is a closed system, oxygen produced during photosynthesis builds up until it inhibits algae growth and, therefore, the algal culture must be returned to a degassing zone where the excess oxygen is removed. In addition, photobioreactors require temperature maintenance and are very difficult and expensive to scale up. Finally, photobioreactors may require periodic cleaning due to biofilm formation and often have dark zones below the outermost algal layer where light intensity is not sufficient to promote algal growth thereby leading to inefficiencies.
In order to overcome some of the noted deficiencies of photobioreactors and open ponds, Applicants have devised the system and methods described herein.